Recently, there have been intense research and development activities on the possible uses of high-T.sub.c -superconductor materials. For example, high-T.sub.c -superconductors were found useful in applications such as semiconductor devices and circuit interconnections. Accordingly, there are now demands for methods for producing uniform high-T.sub.c -superconductor layers combining a satisfactory quality with high efficiency.
Conventional methods for the manufacture of compounds, especially superconductors consisting of three base materials, so-called ternary compounds, are described in a great number of publications, e.g. EP-A-0450394, EP-A-0444698, EP-A-0299870, EP-A-0426570 and EP-A-0446789. Films of high-T.sub.c -superconductor material are usually deposited on a suitable substrate, in most cases on the clean surface of a single crystal. Often the substrate is additionally covered by a buffer layer to avoid interactions between substrate and deposited layers and to facilitate crystal growth in a defined orientation. Deposited onto said substrate or buffer layer are layers of base material, selected to form the high-T.sub.c -superconductor, using a variety of techniques like sputtering, laser ablation, co-evaporation and monolayer-by-monolayer deposition. These deposition techniques can roughly be divided into two different categories: either all materials necessary to build up the unit cell of the compound are deposited simultaneously, or the different base materials are deposited consecutively by piling up monolayers of the different base materials. As understood herein, a unit cell of an ideal crystal is an identical volume which completely fills the space of the crystal without any overlapping. A monolayer is the amount of an element or compound necessary to cover the entire surface of a substrate with a layer of single atoms or molecules, respectively.
Both deposition techniques, as described above, are not satisfactory with regard to impurities and defects of the resulting high-T.sub.c -superconductor material. These faults cause an undesired decrease in the mechanical and electrical properties of the grown crystal structure of the superconductor.
The multilayer deposition technique is known from various publications (Y. Sorimachi et al., Jap. Journal of Appl. Phys.26(9), 1987, pp.L1451-L1452; B.-Y. Tsaur et al., Appl. Phys. Lett. 51(11), 1987, pp.858-860; Z. L. Bao et al., Appl. Phys. Lett. 51(12), 1987, pp.946-947; C.X. Qiu et al., Appl. Phys. Lett. 52(7), 1988, pp.587-589; N. Hess et al., Appl. Phys. Lett. 53(8), 1988, pp.698-699; X. K. Wang et al., Appl. Phys. Lett. 54(16), 1989, pp.1573-1575). This technique is characterized by sequentially depositing layers of suitable base materials onto a heated substrate. In contrast to the monolayer-by-monolayer technique, the thickness of the layers is determined by the stoichiometric ratio of the compound to be produced. Usually the thickness of a single layer lies within a range from 8 nm to 100 nm, corresponding to 30 to over 300 monolayers. The layers of different base materials are combined to a period of layers, a number of which may be repeatedly deposited until the desired thickness of the film is reached. The known multilayer deposition of thick layers requires an annealing step, during which the deposited layers are heated in an oxygen atmosphere. This annealing step is necessary to balance the deficiency in the oxygen content of the deposited layers to transform these layers into superconducting material. As the temperature of the substrate has to be increased substantially by more than 100 Kelvin, and often the vacuum chamber has to be changed to proceed, the known multilayer deposition technique is a relatively slow process. The annealing step further prevents an in-situ control over the growth of the desired compound.
It is, therefore, an object of the present invention to overcome the above-mentioned problems and to provide a new process based on the multilayer deposition to manufacture thin films of solid compounds having crystal structure with improved qualities. Another object of the invention is to provide a method for producing films of high-T.sub.c -superconducting material showing an homogeneous crystal structure. It is a further object of the invention to provide a method for producing films having a smooth surface. Another important object of the invention is to eliminate the previously required annealing step from the manufacturing process of high-T.sub.c -superconducting material.